1. Technical Field
This invention relates to alumina supported cobalt catalysts having an improved stability and high activity for Fischer-Tropsch (F-T) synthesis conducted in a slurry bubble column and other three-phase type reactors.
2. Background
In Fischer-Tropsch processes, synthesis gases comprising carbon oxides and hydrogen are reacted in the presence of Fischer-Tropsch catalysts to produce liquid hydrocarbons. Fischer-Tropsch synthesis processes are most commonly conducted in fixed bed, gas-solid or gas-entrained fluidized bed reaction systems, fixed bed reaction systems being the most commonly used. It is recognized in the art, however, that slurry bubble column reactor systems offer tremendous potential benefits over these commonly used Fischer-Tropsch reaction systems.
As mentioned above, the synthesis gas, or "syngas," used in Fischer-Tropsch processes is typically a mixture consisting primarily of hydrogen and carbon oxides. Syngas is typically produced, for example, during coal gasification. Processes are also well known for obtaining syngas from other hydrocarbons, including natural gas. U.S. Pat. No. 4,423,265 to Chu et al. notes that the major processes for producing syngas depend either upon the partial combustion of a hydrocarbon fuel with an oxygen-containing gas or the reaction of the fuel with steam, or on a combination of these two reactions. U.S. Pat. No. 5,324,335 to Benham et al., explains the two primary methods (i.e., steam reforming and partial oxidation) for producing syngas from methane. The Encyclopedia of Chemical Technology, Second Edition, Volume 10, pages 3553-433 (1966), Interscience Publishers, New York, N.Y. and Third Edition, Volume 11, pages 410-446 (1980), John Wiley and Sons, New York, N.Y. is said by Chu et al. to contain an excellent summary of gas manufacture, including the manufacture of synthesis gas.
It has long been recognized that syngas can be converted to liquid hydrocarbons by the catalytic hydrogenation of carbon monoxide. The general chemistry of the Fischer-Tropsch synthesis process is as follows: EQU CO+2H.sub.2.fwdarw.(--CH.sub.2 --)+H.sub.2 O (1) EQU 2CO+H.sub.2.fwdarw.(--CH.sub.2 --)+CO.sub.2 (2)
The types and amounts of reaction products, i.e., the lengths of carbon chains, obtained via Fischer-Tropsch synthesis vary dependent upon process kinetics and the catalyst selected.
Many attempts at providing active catalysts for selectively converting syngas to liquid hydrocarbons have previously been disclosed. U.S. Pat. No. 5,248,701 to Soled et al., presents an over-view of relevant prior art. The two most popular types of catalysts heretofore used in Fischer-Tropsch synthesis have been iron-based catalysts and cobalt-based catalysts. U.S. Pat. No. 5,324,335 to Benham et al. discusses the fact that iron-based catalysts, due to their high water gas shift activity, favor the overall reaction shown in (2) above, while cobalt-based catalysts tend to favor reaction scheme (1).
Recent advances have provided a number of catalysts active in Fischer-Tropsch synthesis. Besides iron and cobalt, other Group VIII metals, particularly ruthenium, are known Fischer-Tropsch catalysts. The current practice is to support such catalysts on porous, inorganic refractory oxides. Particularly preferred supports include silica, alumina, silica-alumina, and titania. In addition, other refractory oxides selected from Groups III, IV, V, VI and VIII may be used as catalyst supports.
The prevailing practice is to also add promoters to the supported catalyst. Promoters can include ruthenium (when not used as the primary catalyst component), rhenium, hafnium, cerium, and zirconium. Promoters are known to increase the activity of the catalyst, sometimes rendering the catalyst three to four times as active as its unpromoted counterpart.
Contemporary cobalt catalysts are typically prepared by impregnating the support with the catalytic material. As described in U.S. Pat. No. 5,252,613 to Chang et al., a typical catalyst preparation may involve impregnation, by incipient wetness or other known techniques, of, for example, a cobalt nitrate salt onto a titania, silica or alumina support, optionally followed or preceded by impregnation with a promoter material. Excess liquid is then removed and the catalyst precursor is dried. Following drying, or as a continuation thereof, the catalyst is calcined to convert the salt or compound to its corresponding oxide(s). The oxide is then reduced by treatment with hydrogen, or a hydrogen-containing gas, for a period of time sufficient to substantially reduce the oxide to the elemental or catalytic form of the metal. U.S. Pat. No. 5,498,638 to Long points to U.S. Pat. Nos. 4,673,993, 4,717,702, 4,477,595, 4,663,305, 4,822,824, 5,036,032, 5,140,050, and 5,292,705 as disclosing well known catalyst preparation techniques.
As also mentioned above, Fischer-Tropsch synthesis has heretofore been conducted primarily in fixed bed reactors, gas-solid reactors, and gas-entrained fluidized bed reactors, fixed bed reactors being the most utilized. U.S. Pat. No. 4,670,472 to Dyer et al. provides a bibliography of several references describing these systems. The entire disclosure of U.S. Pat. No. 4,670,472 is incorporated herein by reference.
In contrast to these other hydrocarbon synthesis systems, slurry bubble column reactors are "three phase" (i.e., solid, liquid, and gas/vapor) reaction systems involving the introduction of a fluidizing gas into a reactor containing catalyst particles slurried in a hydrocarbon liquid. The catalyst particles are slurried in the liquid hydrocarbons within a reactor chamber, typically a tall column. Syngas is then introduced at the bottom of the column through a distributor plate, which produces small gas bubbles. The gas bubbles migrate up and through the column, causing beneficial agitation and turbulence, while reacting in the presence of the catalyst to produce liquid and gaseous hydrocarbon products. Gaseous products are captured at the top of the SBCR, while liquid products are recovered through a filter which separates the liquid hydrocarbons from the catalyst fines. U.S. Pat. Nos. 4,684,756, 4,788,222, 5,157,054, 5,348,982, and 5,527,473 reference this type of system and provide citations to pertinent patent and literature art. The entire disclosure of each of these patents is incorporated herein by reference.
It is recognized that conducting Fischer-Tropsch synthesis using a SBCR system could provide significant advantages. As noted by Rice et al. in U.S. Pat. No. 4,788,222, the potential benefits of a slurry process over a fixed bed process include better control of the exothermic heat produced by the Fischer-Tropsch reactions, as well as better maintenance of catalyst activity by allowing continuous recycling, recovery and rejuvenation procedures to be implemented. U.S. Pat. Nos. 5,157,054, 5,348,982, and 5,527,473 also discuss advantages of the SBCR process.
Normal operation of F-T synthesis leads to the buildup of carbonaceous deposits on a cobalt catalyst resulting in catalyst deactivation with time-on-stream, the amount of this major source of deactivation being related to the reaction conditions used. In general, cobalt catalysts may be regenerated by calcination at relatively high temperatures (burning off the carbon residues) followed by reduction. However, the successive exposure of these catalysts to high temperatures may result in a slow decrease of the support surface area followed by encapsulation of cobalt particles and the formation of harder to reduce or totally non-reducible cobalt-metal compounds. All these changes are associated with a decrease of the cobalt surface area accessible to reactants which results in a slow loss of activity after each regeneration cycle.
Alumina, one of the common oxides used as a support for cobalt-based F-T catalysts, is well known to be sensitive to the pretreatment temperatures and the amount of time it is subjected to high temperatures. The crystalline form of the alumina most commonly used as catalyst support is .gamma.-alumina. It is generally obtained by dehydration of aluminum hydroxide (boehmite) by heating under suitable conditions (typically, 300-650.degree. C.). Further heating, either during the pretreatment step, during the use of the catalyst or during catalyst regeneration may result in a slow and continuous loss of surface area and a slow conversion of the alumina from its .gamma.-alumina phase to other forms (.delta.-alumina then .theta.-alumina) which have much lower surface areas. Finally, especially at very high temperatures, a collapse of the structure resulting in the formation of a dense, highly stable, low surface area .alpha.-alumina can occur.
It has been suggested by Condea/Vista and by R. Gaugin, M. Graulier, and D. Papee, "Thermally Stable Carriers," Advances in Chemistry Series, Vol. 143, p. 147 (1975) that the thermal stability of some .gamma.-aluminas materials can be enhanced by incorporating into the alumina small amounts of divalent ions, such as calcium, magnesium, or barium or rare earth oxides such as lanthana. These are believed to occupy tetrahedral voids in the spinel and retard the diffusion of Al.sup.3+ cations. However, the effects of such support additives on the activities and other characteristics of any catalysts formed therefrom are unknown. The activities and selectivities of Fischer-Tropsch catalysts, for example, are known to be extremely sensitive to changes in catalyst or support compositions.